Took some more test luminance frames on 2016-07-28, 2016-08-02 and 2016-10-03 to decide upon gain, offset and exposure time. Added RGB from previous DSLR image of same object, see http://astrophotos.se/?p=600

66x30s+17x60s+9x90s+19x120s+8x180s+26x240s+3x300s exposures with ASI1600MM-COOL and IDAS LPS-D1 filter used for luminance (224.5 minutes total exposure) taken with gain 77, offset 12 and gain 100, offset 17 settings of the camera cooled to -20C. 16x300s+8x240s+9x180s exposures with Canon EOS600Da used for RGB (139 minutes total exposure for RGB)

With gain 77 setting, the 120s exposures seem to be optimal (with some caveat since the sky was not fully dark). With gain 100, offset 17 I could not see any difference in amount and detail of nebulosity and stars when stacking 23 30s exposures compared to 3 240s exposures apart from the 30s stack had less noise and less saturated stars. This suggests that 30s exposures could be used.